Muxers
******

Muxers are configured elements in Libav which allow writing multimedia streams to a particular type of file.

When you configure your Libav build, all the supported muxers are enabled by default. You can list all available muxers using the configure option --list-muxers.

You can disable all the muxers with the configure option --disable-muxers and selectively enable / disable single muxers with the options --enable-muxer=MUXER / --disable-muxer=MUXER.

The option -formats of the ff* tools will display the list of enabled muxers.

A description of some of the currently available muxers follows.

crc
===

CRC (Cyclic Redundancy Check) testing format.

This muxer computes and prints the Adler-32 CRC of all the input audio and video frames. By default audio frames are converted to signed 16-bit raw audio and video frames to raw video before computing the CRC.

The output of the muxer consists of a single line of the form: CRC=0xCRC, where CRC is a hexadecimal number 0-padded to 8 digits containing the CRC for all the decoded input frames.

For example to compute the CRC of the input, and store it in the file out.crc:

     avconv -i INPUT -f crc out.crc

You can print the CRC to stdout with the command:

     avconv -i INPUT -f crc -

You can select the output format of each frame with avconv by specifying the audio and video codec and format. For example to compute the CRC of the input audio converted to PCM unsigned 8-bit and the input video converted to MPEG-2 video, use the command:

     avconv -i INPUT -c:a pcm_u8 -c:v mpeg2video -f crc -

See also the framecrc muxer.

framecrc
========

Per-frame CRC (Cyclic Redundancy Check) testing format.

This muxer computes and prints the Adler-32 CRC for each decoded audio and video frame. By default audio frames are converted to signed 16-bit raw audio and video frames to raw video before computing the CRC.

The output of the muxer consists of a line for each audio and video frame of the form: STREAM_INDEX, FRAME_DTS, FRAME_SIZE, 0xCRC, where CRC is a hexadecimal number 0-padded to 8 digits containing the CRC of the decoded frame.

For example to compute the CRC of each decoded frame in the input, and store it in the file out.crc:

     avconv -i INPUT -f framecrc out.crc

You can print the CRC of each decoded frame to stdout with the command:

     avconv -i INPUT -f framecrc -

You can select the output format of each frame with avconv by specifying the audio and video codec and format. For example, to compute the CRC of each decoded input audio frame converted to PCM unsigned 8-bit and of each decoded input video frame converted to MPEG-2 video, use the command:

     avconv -i INPUT -c:a pcm_u8 -c:v mpeg2video -f framecrc -

See also the crc muxer.

image2
======

Image file muxer.

The image file muxer writes video frames to image files.

The output filenames are specified by a pattern, which can be used to produce sequentially numbered series of files. The pattern may contain the string "%d" or "%0Nd", this string specifies the position of the characters representing a numbering in the filenames. If the form "%0Nd" is used, the string representing the number in each filename is 0-padded to N digits. The literal character '%' can be specified in the pattern with the string "%%".

If the pattern contains "%d" or "%0Nd", the first filename of the file list specified will contain the number 1, all the following numbers will be sequential.

The pattern may contain a suffix which is used to automatically determine the format of the image files to write.

For example the pattern "img-%03d.bmp" will specify a sequence of filenames of the form img-001.bmp, img-002.bmp, ..., img-010.bmp, etc. The pattern "img%%-%d.jpg" will specify a sequence of filenames of the form img%-1.jpg, img%-2.jpg, ..., img%-10.jpg, etc.

The following example shows how to use avconv for creating a sequence of files img-001.jpeg, img-002.jpeg, ..., taking one image every second from the input video:

     avconv -i in.avi -vsync 1 -r 1 -f image2 'img-%03d.jpeg'

Note that with avconv, if the format is not specified with the -f option and the output filename specifies an image file format, the image2 muxer is automatically selected, so the previous command can be written as:

     avconv -i in.avi -vsync 1 -r 1 'img-%03d.jpeg'

Note also that the pattern must not necessarily contain "%d" or "%0Nd", for example to create a single image file img.jpeg from the input video you can employ the command:

     avconv -i in.avi -f image2 -frames:v 1 img.jpeg

MOV/MP4/ISMV
============

The mov/mp4/ismv muxer supports fragmentation. Normally, a MOV/MP4 file has all the metadata about all packets stored in one location (written at the end of the file, it can be moved to the start for better playback using the qt-faststart tool). A fragmented file consists of a number of fragments, where packets and metadata about these packets are stored together. Writing a fragmented file has the advantage that the file is decodable even if the writing is interrupted (while a normal MOV/MP4 is undecodable if it is not properly finished), and it requires less memory when writing very long files (since writing normal MOV/MP4 files stores info about every single packet in memory until the file is closed). The downside is that it is less compatible with other applications.

Fragmentation is enabled by setting one of the AVOptions that define how to cut the file into fragments:

-movflags frag_keyframe
     Start a new fragment at each video keyframe.

-frag_duration DURATION
     Create fragments that are DURATION microseconds long.

-frag_size SIZE
     Create fragments that contain up to SIZE bytes of payload data.

-movflags frag_custom
     Allow the caller to manually choose when to cut fragments, by calling av_write_frame(ctx, NULL) to write a fragment with the packets written so far. (This is only useful with other applications integrating libavformat, not from avconv.)

Additionally, the way the output file is written can be adjusted through a few other options:

-movflags empty_moov
     Write an initial moov atom directly at the start of the file, without describing any samples in it. Generally, an mdat/moov pair is written at the start of the file, as a normal MOV/MP4 file, containing only a short portion of the file. With this option set, there is no initial mdat atom, and the moov atom only describes the tracks but has a zero duration.

     Files written with this option set do not work in QuickTime. This option is implicitly set when writing ismv (Smooth Streaming) files.

-movflags separate_moof
     Write a separate moof (movie fragment) atom for each track. Normally, packets for all tracks are written in a moof atom (which is slightly more efficient), but with this option set, the muxer writes one moof/mdat pair for each track, making it easier to separate tracks.

     This option is implicitly set when writing ismv (Smooth Streaming) files.

Smooth Streaming content can be pushed in real time to a publishing point on IIS with this muxer. Example:

     avconv -re <NORMAL INPUT/TRANSCODING OPTIONS> -movflags isml+frag_keyframe -f ismv http://server/publishingpoint.isml/Streams(Encoder1)

mpegts
======

MPEG transport stream muxer.

This muxer implements ISO 13818-1 and part of ETSI EN 300 468.

The muxer options are:

-mpegts_original_network_id NUMBER
     Set the original_network_id (default 0x0001). This is unique identifier of a network in DVB. Its main use is in the unique identification of a service through the path Original_Network_ID, Transport_Stream_ID.

-mpegts_transport_stream_id NUMBER
     Set the transport_stream_id (default 0x0001). This identifies a transponder in DVB.

-mpegts_service_id NUMBER
     Set the service_id (default 0x0001) also known as program in DVB.

-mpegts_pmt_start_pid NUMBER
     Set the first PID for PMT (default 0x1000, max 0x1f00).

-mpegts_start_pid NUMBER
     Set the first PID for data packets (default 0x0100, max 0x0f00).

The recognized metadata settings in mpegts muxer are service_provider and service_name. If they are not set the default for service_provider is "Libav" and the default for service_name is "Service01".

     avconv -i file.mpg -c copy \
          -mpegts_original_network_id 0x1122 \
          -mpegts_transport_stream_id 0x3344 \
          -mpegts_service_id 0x5566 \
          -mpegts_pmt_start_pid 0x1500 \
          -mpegts_start_pid 0x150 \
          -metadata service_provider="Some provider" \
          -metadata service_name="Some Channel" \
          -y out.ts

null
====

Null muxer.

This muxer does not generate any output file, it is mainly useful for testing or benchmarking purposes.

For example to benchmark decoding with avconv you can use the command:

     avconv -benchmark -i INPUT -f null out.null

Note that the above command does not read or write the out.null file, but specifying the output file is required by the avconv syntax.

Alternatively you can write the command as:

     avconv -benchmark -i INPUT -f null -

matroska
========

Matroska container muxer.

This muxer implements the matroska and webm container specs.

The recognized metadata settings in this muxer are:

title=TITLE NAME
     Name provided to a single track

language=LANGUAGE NAME
     Specifies the language of the track in the Matroska languages form

STEREO_MODE=MODE
     Stereo 3D video layout of two views in a single video track

     mono
          video is not stereo

     left_right
          Both views are arranged side by side, Left-eye view is on the left

     bottom_top
          Both views are arranged in top-bottom orientation, Left-eye view is at bottom

     top_bottom
          Both views are arranged in top-bottom orientation, Left-eye view is on top

     checkerboard_rl
          Each view is arranged in a checkerboard interleaved pattern, Left-eye view being first

     checkerboard_lr
          Each view is arranged in a checkerboard interleaved pattern, Right-eye view being first

     row_interleaved_rl
          Each view is constituted by a row based interleaving, Right-eye view is first row

     row_interleaved_lr
          Each view is constituted by a row based interleaving, Left-eye view is first row

     col_interleaved_rl
          Both views are arranged in a column based interleaving manner, Right-eye view is first column

     col_interleaved_lr
          Both views are arranged in a column based interleaving manner, Left-eye view is first column

     anaglyph_cyan_red
          All frames are in anaglyph format viewable through red-cyan filters

     right_left
          Both views are arranged side by side, Right-eye view is on the left

     anaglyph_green_magenta
          All frames are in anaglyph format viewable through green-magenta filters

     block_lr
          Both eyes laced in one Block, Left-eye view is first

     block_rl
          Both eyes laced in one Block, Right-eye view is first

For example a 3D WebM clip can be created using the following command line:

     avconv -i sample_left_right_clip.mpg -an -c:v libvpx -metadata STEREO_MODE=left_right -y stereo_clip.webm

segment
=======

Basic stream segmenter.

The segmenter muxer outputs streams to a number of separate files of nearly fixed duration. Output filename pattern can be set in a fashion similar to image2.

Every segment starts with a video keyframe, if a video stream is present. The segment muxer works best with a single constant frame rate video.

Optionally it can generate a flat list of the created segments, one segment per line.

segment_format FORMAT
     Override the inner container format, by default it is guessed by the filename extension.

segment_time T
     Set segment duration to T seconds.

segment_list NAME
     Generate also a listfile named NAME.

segment_list_size SIZE
     Overwrite the listfile once it reaches SIZE entries.

segment_wrap LIMIT
     Wrap around segment index once it reaches LIMIT.

     avconv -i in.mkv -c copy -map 0 -f segment -list out.list out%03d.nut

mp3
===

The MP3 muxer writes a raw MP3 stream with an ID3v2 header at the beginning and optionally an ID3v1 tag at the end. ID3v2.3 and ID3v2.4 are supported, the id3v2_version option controls which one is used. The legacy ID3v1 tag is not written by default, but may be enabled with the write_id3v1 option.

For seekable output the muxer also writes a Xing frame at the beginning, which contains the number of frames in the file. It is useful for computing duration of VBR files.

The muxer supports writing ID3v2 attached pictures (APIC frames). The pictures are supplied to the muxer in form of a video stream with a single packet. There can be any number of those streams, each will correspond to a single APIC frame. The stream metadata tags TITLE and COMMENT map to APIC DESCRIPTION and PICTURE TYPE respectively. See http://id3.org/id3v2.4.0-frames for allowed picture types.

Note that the APIC frames must be written at the beginning, so the muxer will buffer the audio frames until it gets all the pictures. It is therefore advised to provide the pictures as soon as possible to avoid excessive buffering.

Examples:

Write an mp3 with an ID3v2.3 header and an ID3v1 footer:

     avconv -i INPUT -id3v2_version 3 -write_id3v1 1 out.mp3

Attach a picture to an mp3:

     avconv -i input.mp3 -i cover.png -c copy -metadata:s:v title="Album cover"
     -metadata:s:v comment="Cover (Front)" out.mp3